Switching device, particularly for optical switching

ABSTRACT

This switching device comprises at least one cell made up of a moving structure consisting in a flexible cantilever beam ( 14 ) and an integral screen ( 16 ) attached to said beam, being able to move between two end positions (A, B) and of actuating electrodes ( 20 ) for subjecting said structure to forces of attraction to control its movements. The electrodes ( 20 ) are located on each side of the beam ( 14 ) in such a way as to follow its exact shape when it is in one of its two end positions. A regular movement of the moving structure and a reliable operating of the switching device are thus obtained This invention may be used in optical switching applications.

This invention is a switching device comprising a cell which includes astructure capable of moving back and forth between two end positions andactuating electrodes to subject the structure to attraction forces tocontrol its movement.

Devices of this type are used, in particular, to influence or interruptthe trajectory of optical beams in systems which control, process andstore information on pixel-based images. One particularly interestingapplication for these systems is for printing or high definitionreproduction systems.

French patent FR 2 726 135 describes one such device, made in the formof a micro-mechanical structure, and an installation for opticalswitching using several devices placed in an array.

This invention aims to improve the reliability, durability andperformance of the above-mentioned device. In particular, the aim of theinvention is to provide a switching device where the cell is equippedwith a moving structure which moves in a regular way in a single plane,does not rebound when it reaches its end positions and where there islittle risk of remaining stuck to the electrodes.

In order to achieve these goals, the switching device according to theinvention is characterised by the actuating electrodes of its cell beingpositioned on both sides of the moving structure, in such a way as tofollow the exact shape of said moving structure when it is at either ofits two end positions.

This device also benefits from one or several of the maincharacteristics listed below.

The moving structure is formed by a flexible cantilever beam and ascreening blade integrally connected to the beam. In this case, theelectrodes are only located on each side of the beam.

Adjacent to each electrode there is at least one stopper serving as astop for the beam and intended to prevent it from coming into contactwith the electrode. This stopper is, for instance, located at the freeend of the cantilever beam, beyond the screen. Furthermore, it ispreferable to provide for several stoppers distributed along the wholelength of the electrodes.

The side walls of the moving structure, stoppers and electrodes arecoated with a layer of diamond, preferably made to act as a conductor,or any material with the same properties.

The switching device is made on a substrate which has a transverseaperture positioned in such a way as to be closed by the screen when thesystem is in one of its end positions. This aperture has, on thesubstrate's side opposite the screen, one portion with a greaterdiameter than its portion located on the side of the screen. It ispreferable if this portion is conical.

The beam has either a ‘T’ shaped cross-section or any other shapedcross-section able to enhance its out-of-plane rigidity.

Each electrode connected to a conductor is connected to a controlcircuit; the connection of the electrode to this conductor may comprisean incorporated fuse.

Other characteristics and advantages of the invention will be shown inthe description below, made with regard to the attached drawings inwhich:

FIG. 1 is a top-view of a switching cell according to the invention;

FIG. 2 is a cross-section according to II—II of the cell in FIG. 1;

FIG. 3 is a cross-section according to III—III of the cell in FIG. 1,and

FIG. 4 is a schematic representation of a four cell array with theirinterconnections.

FIGS. 1 to 3 show a substrate 10, preferably made from silicon, on whichis attached, by means of an anchoring area 12, an elastic cantileverbeam 14 with, in this particular embodiment, a ‘T’ shaped cross-sectionwith, on its free end, a rectangular screen 16. It is preferable thatthe beam 14 and screen 16 are made of polysilicon made to act as aconductor by doping.

The beam 14 and screen 16 make up a moving structure, the screen beingable to be moved alternately between the two end positions, A and Brespectively, represented by solid and dotted lines. These positions aredefined by two end stoppers 18, located on the substrate 10, which actto stop the beam 14.

The screen is brought to these end positions under the effect ofelectrostatic forces generated by a pair of addressing electrodes 20,made of doped polysilicon and attached to the substrate by an isolatinglayer 22, on both sides of the beam 14 and along its length, in such away as to follow its exact shape as closely as possible without,however, touching it, when the screen occupies positions A and Brespectively.

Preferably, the isolating layer 22 is formed by a coating of silicondioxide (SiO2) deposited on the substrate and a layer of silicon nitride(Si3N4) deposited on the dioxide coating.

The electrodes 20 have, along their length, zones within which arelocated pairs of stoppers 24, which may, for instance, be identical tothe end stoppers 18, one of the five in the example given, electricallyisolated from the electrodes and intended to limit movements of the beamso that it does not come into contact with the electrodes.

It is preferable if the side walls of the beam 14, stoppers 18, 24 andelectrodes 20 are coated with a diamond layer made to be hydrophobic andconducting by means of an appropriate doping treatment, known by thoseskilled in the art. As well as diamond, any other material which has thesame properties or such other properties as to avoid in-use stiction,particularly a polymer, self-assembled mono-layers, teflon or alike mayalso be used for this coating.

At the centre of the portion closed by the screen 16 when it is inposition A, the substrate 10 has a transverse aperture 26 which, on thescreen side, has a preferably cylindrical portion 26 a of a slightlysmaller diameter to that of the screen side, and a gradually wideningportion 26 b which is preferably conical. It should be noted thatinstead of being rectangular, the screen might be of any shape and sizewhich makes it capable to cover the aperture when the beam is in itsclosed position.

According to an advantageous embodiment of the invention, the substrate10 is made of a transparent material, such as quartz.

In its central portion, above which the beam 14 and the screen 16 move,the substrate is coated with a doped polysilicon layer 28 electricallyconnected to the beam, stoppers and screen in such a way as to have thesame potential and to thus prevent any attraction between them.

The cell described is preferably not used individually, but inassociation with other identical cells arranged in lines and columns andforming a regular array which, as already known in the art, may be,among other arrangements, rectangular, honeycombed or hexagonal.

FIG. 4 shows, schematically, four identical cells C1, C2, C3 and C4,located in an array made up of two rows and two columns. The beam 14,the screen 16 and the pair of electrodes 20 can be found.

The anchoring areas 12 of cells C1 and C2 on the upper row are connectedto a common line conductor 30. In the same way, the anchoring areas 12of cells C3 and C4 on the lower row are connected to another common lineconductor 32.

The right-hand electrodes 20 of the right-hand cells C2 and C4 areconnected to a common right-hand column conductor 34, while theleft-hand electrodes 20 of these cells are connected to a commonleft-hand column conductor 38. In the same way, the right-handelectrodes 20 of the left-hand cells C1 and C3 are connected to anothercommon column conductor 36, while the left-hand electrodes 20 of thesecells are connected to another common column conductor 40.

For details on how to control cells in this type of array, reference ismade to the above-mentioned patent FR 2 726 135.

It is known that, despite the precautions taken, and in particular, thepresence of stoppers 18, 24, there may be a risk of short-circuit,caused, in particular, by the beam 14 coming into contact with anelectrode 20. In order to prevent such an accident from causing thefailure of all the cells in the same line and or column, which can occurin known systems, it is proposed that the connection of each of theelectrodes 20 with its column conductor to be made, as shown in FIG. 4,by the intermediary of a fuse 42 which, for instance, could be made inthe form of a narrow section of the connection line.

A switching cell is thus produced for which the main advantages whencompared to known embodiments, particularly as described in patent FR 2726 135, are listed below:

Due to the fact that, on the one hand, the electrodes are positioned insuch a way as to exactly match the beam when it is in either of its twoend positions and that, on the other hand, the screen itself is notsubjected to the direct action of the electrodes, the movement of themoving structure becomes regular, in a single plane, and is free from—orat least has limited—rebounds.

The presence of stoppers distributed along the length of the electrodescontributes equally to this operation in a smooth way while at the sametime reducing the risk of the beam sticking to the stoppers.

The structured section of the beam prevents out-of-plane bending andtherefore also contributes to the system always moving parallel to thesubstrate.

The presence of a conductive diamond coating on the side walls asregards the beam, electrodes and stoppers prevents the accumulation ofcharges that cause the beam to stick to the stoppers.

The fuse located between each electrode and the conductor of theassociated column and/or line will limit the consequences of ashort-circuit occurring in a cell.

Finally, the conical section of the aperture in the substrate preventsloss of light and significantly improves the optical transmission of thedevice.

What is claimed is:
 1. A switching device comprising at least one cellmade up of a moving structure capable of moving between two endpositions and fixed actuating electrodes for subjecting said movingstructure to forces of attraction to control its movement, said movingstructure being movable relative to said electrodes and electrodes beinglocated on each side of the moving structure in such a way as to followits exact shape when it is in one of its two end positions, said movingstructure comprising a flexible cantilever beam and an integral screenattached to said beam, wherein said electrodes are located only on eachside of said beam.
 2. The switching device of claim 1, furthercomprising, associated with each electrode, at least one stopper servingas a stop for said beam and for the purpose of preventing said beamcoming into contact with said electrode.
 3. The switching device ofclaim 2, said stopper being located at the free end of said cantileverbeam, beyond said screen.
 4. The switching device of claim 2, furthercomprising a plurality of stoppers distributed along the length of saidelectrodes.
 5. The switching device of claim 2, wherein the side wallsof the moving structure, stoppers and electrodes are coated with adiamond layer.
 6. The switching device of claim 5, characterized in thatsaid diamond layer is conductive.
 7. The switching device of claim 1,said switching device being made on a substrate.
 8. The switching deviceof claim 7, said substrate being made of transparent material.
 9. Theswitching device of claim 7, said substrate having a transverse aperturepositioned in such a way as to be closed by the screen when the systemis in one of its end positions.
 10. The switching device of claim 9,said aperture, on the opposite side of the screen, having a portion of agreater diameter than the portion located on the side of the screen. 11.The switching device of claim 10, said portion of greater diameter beingconical.
 12. The switching device of claim 1, said beam presenting a ‘T’shaped section.
 13. The switching device of claim 1, each electrodebeing connected to a conductor linked to a control circuit, theelectrode connection with said conductor comprising a fuse.